Depends on what one values. I tried TST and Sidebery among other extensions for Firefox, but their "bolted on" feeling always bothered me. That can be somewhat fixed with userChrome mods, but those tend to be fragile and it's not unusual for them break with updates, and even with the mods they're not as integrated as native vertical tabs.
For my personal usage I find the deep nesting of TST to get in the way more than it helps and so the simpler model of Arc/Zen where nesting only happens intentionally is nice.
Indeed you need to write some firefox css to use either TST or Sidebery. But it's pretty rare for it to break. It actually broke a few days ago, I fixed it in a few minutes - ctrl+shift+alt+i opens the developer tools for the browser UI itself, then it's a matter of selecting the faulty element and writing the CSS to hide it or whatever. Or, ask a LLM to write the CCS for you. Or check out https://www.reddit.com/r/FirefoxCSS/ since probably someone has posted a fix there
> even with the mods they're not as integrated as native vertical tabs.
I was maintaining such a setup for a while and didn't have such luck. Sometimes it'd fully break and sometimes things would shift around due to other CSS changing, but whatever the case it's more than I had the patience to babysit.
> Hmm what's Sidebery missing?
The extensions themselves aren't lacking, but they suffer from being extensions and not part of the browser proper, which makes for occasional weird interactions and glitches and the "seams" between extension and host becoming tangible from time to time. Things just work better when the browser "knows" about the tabs being vertical and having deeper organization than is possible with horizontal tab bars, or is even designed around vertical tabs being the only option (like Arc/Zen).
For my personal usage I find the deep nesting of TST to get in the way more than it helps and so the simpler model of Arc/Zen where nesting only happens intentionally is nice.